This invention relates to polymeric bodies having a multilayered structure, and more particularly relates to multilayered thermoplastic tubular parisons formed by sealing off lengths of tubing, the tubing being prepared by coextrusions techniques having a low-gas permeable layer situated over substantially the entire area thereof. Moreover, the subject invention relates to a method for closing or sealing an open end of multilayered tube bodies having inner and outer thermoplastic layers with a barrier layer therebetween, the sealing being done to assure the integrity of the barrier layer over the full extent of the sealed off portions.
The formation of multilayered structures is known wherein the separate layers contribute in some degree to the final properties. Many types of multilayered structures can be prepared by melting each individual component in a separate zone and subsequently combining them in a predetermined configuration. In general, multilayered structures of two or more different polymeric materials may be readily coextruded or cojoined to form varied shaped multilayered structures. In particular, multilayered structures of a tubular or cylindrical configuration may be readily made by conventional coextrusion processes wherein each thermoplastic material which ultimately forms a layer in the multilayered structure is separately heated to its respective melt-extrusion temperature by a conventional melt extruder and, thereafter, each melted material is forced by pressure-feeding means into streams of melted thermoplastic material that enter a multimanifold die device provided with toroidal chambers and a common annular die orifice. The melted materials are fed at the same time into said device and form therein continuous annular layers as they exit the die orifice, each additional layer being fed to the device and issuing as annular coaxial layers that are essentially concentric with the initially formed annular layer. In essence, the device provides an annular flow of plastic materials, one strata of thermoplastic material being circumferentially deposited upon the preceding one to produce the multilayered tubular body. After being formed and subsequently cooled, the tubular body in the form of continuous tubing is cut into predetermined lengths or parisons blanks that are in effect cylindrical multilayered structure having two open ends. In attempting to seal or close off an open end of a multilayered structure, especially structures having at least one low-gas permeable layer therein, it is soon realized that it is most difficult to properly seal the open end portion and still maintain the integrity of the low-gas permeable layer. Generally there is little problem in completely sealing or closing off a monolithic tubular structure, that is, a structure containing only one polymeric material. In such a situation the portion to be sealed or closed off is merely heated and the sidewalls are forced together into sealing contact by means of coacting bearing elements. This may be readily accomplished by simply pinching off and cross-bonding a fusible thermoplastic material, usually along a longitudinal plane. In such a situation the plastic material readily bonds or adheres to itself. It is readily apparent that when the tubular body comprises a multilayered structure wherein certain layers possess certain desired properties that must be evenly distributed throughout the total area there is realized a number of problems that manifest themselves in attempting to simply form and close one end of such a structure. Since there are often a number of different or diverse polymeric materials that are cojoined in the tubular structure, there is a marked tendency during any heating and subsequent severing or cutting of these layers of cojoined materials to constrict or shrink at different rates during the cooling thereof whereby an uneven tubular structure often results. Moreover, since some of the polymeric materials making up the structure have different physical properties such as being less viscous and workable than other plastics at a given temperature, different polymeric materials offer various degrees of formability and resistance and are more difficult to manipulate subsequently in a reforming process within the composite structure. The property of formability is particularly noticeable with barrier or low-gas permeable materials since these materials generally have higher melting points, different backing arrangements within their molecular lattices, greater bond strengths between adjacent molecules and other properties which all play a role in making them more difficult to work and to reform when compared to other thermoplastic materials.
Where low-gas permeable layers are incorporated as a layer or layers in tubular structures there is a marked improvement in achieving uniformity of such a layer or layers throughout all areas of a fabricated tube in following the teachings of this invention. This is of particular importance in the early stage of the tube design and does not matter whether such tube be in a parison form or in a final container form. The subject method offers a process of sealing or completely closing off an open end of a multilayered tube having inner and outer thermoplastic layers with a low-gas permeable layer therebetween whereby the latter layer is assured of its integrity throughout the tube, especially the closed end or basal portion thereof. Furthermore, the subject invention provides a method of utilizing multilayered tubular structures having low-gas permeability as well as containers formed from said tubular structures and bringing about bottom wall sealability which, in turn, enables said structures to be useful for many purposes in the art, especially for parisons in blow molding containers as hereinafter set forth.
Although the particular problems resolved by the subject invention have not been appreciated or squarely addressed by the prior art, there are assuredly related and pertinent patents in this field. As a brief overview, a number of patents disclose methods of making tubular, multilayered structures as well as methods of blow molding certain thermoplastic structures. For example, the method of coextruding multilayered tubular bodies is described in U.S. Pat. No. 3,308,508 to Schrenk as well as U.S. Pat. No. 3,354,506 to Raley. A method for the sealing and severing of parisons is disclosed in U.S. Pat. No. 3,430,290 to Kinslow, Jr., as well as U.S. Pat. No. 3,686,379 to Gilbert. Lastly, U.S. Pat. No. 3,955,697 to Valyi describes a method of making a multilayered, hollow plastic container employing injection molding techniques.